Cell surface proteoglycans are key players in epithelial morphogenesis. They form gradients that immobilize mitogens in proximity to signaling receptors (Wang and Laurie, 2004; Häcker et al., 2005; Radtke and Clevers, 2005), contribute to cellular adhesion by ligating the extracellular matrix (ECM) and at least in one case participate in integrin coupling (Beauvais et al., 2004; McQuade et al., 2006). Cell surface proteoglycans consist of a core protein and associated glycosaminoglycan chains, mainly heparan sulfate (HS). Current dogma states that mitogen, cytokine and ECM binding is largely the domain of the anionic HS chains (Couchman 2003; Häcker et al., 2005). HS chains are generated by a complement of Golgi polymerases, epimerase, and sulphotransferases during post-translational modification. Each is thought to vary in relative activity by cell or tissue type (Perrimon and Bernfield, 2000). Thus within a given epithelium or endothelium, a structurally similar HS chain can be attached to genetically distinct core protein (Zako et al., 2003).
New work has shed light on how HS proteoglycan specificity is generated in development and disease. Most involve extracellular enzymes that affect cell surface HS proteoglycans in unexpected ways. Removal of certain HS 6-O-sulfates by endo-6-O-sulfatases Sulf1 and Sulf2 disrupts the binding of the BMP inhibitor Noggin, leading to its dispersal and establishment of BMP signaling (Viviano et al., 2004). In contrast, this same HS modification diminishes FGF binding and assembly with its signaling receptor (Dai et al., 2005). In another extracellular modification mechanism, HS cleavage by heparanase generates soluble fragments of HS that form complexes of FGF-HS and trigger cellular proliferation, migration, and angiogenesis (Kato et al., 1998). In another mechanism, matrix metalloproteinase-7-dependent shedding of the entire syndecan ectodomain promotes cancer-associated upregulation of glypican-1 and tumor growth (Ding et al., 2005).
In addition to its HS-dependent signaling mechanisms, recent work has shown that the syndecan core proteins themselves participate as cell surface receptors. Their extracellular protein domains regulate the activation of integrins (Beauvais et al. 2003; Beauvais et al., 2004; McQuade et al. 2006), bind growth factors, including Wnt, midkine and pleitrophin (Capurro et al, 2005; Deepa et al. 2004), and disrupt carcinoma activity when added as recombinant competitors, presumably by disrupting their assembly with other signaling receptors at the cell surface.
There is a long felt need in the art for methods and compositions to identify and regulate the signaling pathways of lacritin and syndecan. The present invention satisfies these needs.